ss. In _Roestelia_ the peridia are large, growing in
company, and splitting longitudinally in many cases, or by a
lacerated mouth. In most instances, the spores are brownish, but
in a splendid species from North America (_Roestelia aurantiaca_,
Peck), recently characterized, they are of a bright orange. If
Oersted is correct in his observations, which await confirmation,
these species are all related to species of _Podisoma_ as a
secondary form of fruit.[k] In the _Roestelia_ of the pear-tree, as
well as in that of the mountain ash, the spermogonia will be found
either in separate tufts on discoloured spots, or associated with
the _Roestelia_, In _Peridermium_ there is very little structural
difference from _Roestelia_, and the species are all found on
coniferous trees. In _Endophyllum_, the peridia are immersed in the
succulent substance of the matrix; whilst in _Graphiola_, there is
a tougher and withal double peridium, the inner of which forms a
tuft of erect threads resembling a small brush.[l]

[Illustration: FIG. 25.--_Helminthosporium molle._]

HYPHOMYCETES.--The predominant feature in the structure of this order
has already been intimated to consist in the development of the
vegetative system under the form of simple or branched threads, on
which the fruit is generated. The common name of mould is applied to
them perhaps more generally than to other groups, although the term is
too vague, and has been too vaguely applied to be of much service in
giving an idea of the characteristics of this order. Leaving the
smaller groups, and confining ourselves to the _Dematiei_ and the
_Mucedines_, we shall obtain some notion of the prevalent structure.
In the former the threads are more or less carbonized, in the latter
nearly colourless. One of the largest genera in _Dematiei_ is
_Helminthosporium_. It appears on decaying herbaceous plants, and on
old wood, forming effused black velvety patches. The mycelium, of
coloured jointed threads, overlays and penetrates the matrix; from
this arise erect, rigid, and usually jointed threads, of a dark brown,
nearly black colour at the base, but paler towards the apex. In most
cases these threads have an externally cortical layer, which imparts
rigidity; usually from the apex, but sometimes laterally, the spores
are produced. Although sometimes colourless, these are most commonly
of some shade of brown, more or less elongated, and divided
transversely by few or many septa. In _Helm